One account of the origin of the term 'jeep' begins when the prototypes were being proven at military bases. The term 'jeep' was used by soldiers for any untried or untested piece of personnel or equipment. In early 1941, Willys-Overland staged a press event in Washington, D.C., having the car demonstrate its prowess by driving up the Capitol steps. Irving 'Red' Hausmann, a test driver on the Willys development team, who had accompanied the car for its testing at Camp Holabird, had heard soldiers there referring to it as a jeep. He was enlisted to go to the event and give a demonstration ride to a group of dignitaries, including Katherine Hillyer, a reporter for the Washington Daily News. When asked by the reporter, Hausmann too called it a Jeep. Hillyer's article appeared in the newspaper on February 20, 1941, with a photo showing a jeep going up the Capitol steps and a caption including the term 'jeep'. This is believed to be the most likely cause of the term being fixed in public awareness. Even though Hausmann did not create or invent the word Jeep, he very well could be the one most responsible for its first news media usage.
For more theories about origins of the word 'jeep', see the origin of the term 'jeep'.